High toughness iron balls and process of making the same

ABSTRACT

The high toughness iron balls for grinding having excellent ductility, wear resistance, and heat resistance are composed of white cast iron having fine eutectic cementite structure formed by destroying the network structure of an eutectic structure in as-cast condition, in which said eutectic cementite has been concentric spherically and uniformly dispersed and arranged in the ball.

United States Patent [191 Sato [ 51 Oct. 29, 1974 HIGH TOUGHNESS IRON BALLS AND PROCESS OF MAKING THE SAME [75] Inventor: Yuichiro Sato, Toyama, Japan a [73] Assignee: Taiheiyo Kinzoku KabushikiKaisha,

Tokyo, Japan [22] Filed: Feb. 16, 1973 [21] Appl. No; 333,270

[30] Foreign Application Priority Data Mar. 6, 1972 Japan 47-22235 [52] US. Cl. 148/2, 29/34 C, 29/l48.4 B,

51/309, 148/12, 148/35 [51] Int. Cl. C2ld 7/14, C2ld 9/36, C22c 37/00 2,462,122 2/1949 Nelson 29/148.4 B X 2,611,690 9/1952 Schneidewind.... 51/309 3,061,209 10/1962 Bard 51/309 X 3,423,250 l/1969 Morizumi et a1. 148/12 X FOREIGN PATENTS OR APPLICATIONS 630,138 10/1949 Great Britain 148/141 837,514 6/1960 Great Britain 51/309 OTHER PUBLICATIONS Foundry, July 1954, pp. 86-89 & 244

Primary Examiner-C. Lovell Attorney, Agent, or Firm-Sughrue, Rothwell, Mion,

Zinn and Macpeak 57 ABSTRACT The high toughness iron balls for grinding having excellent ductility, wear resistance, and heat resistance are composed of white cast iron having fine eutectic cementite structure formed by destroying the network structure of an eutectic structure in as-cast condition, in which said eutectic cementite has been concentric spherically and uniformly 'dispersed and arranged in the ball.

2 Claims, 7 Drawing Figures FEW? 2 m I SW? 1 3 gso 0 w U S Pmmsnm 29 m4 SHEET 2 BF 2 FIG. 2b

FIG. '20

FIG. 3b

FIG. 30

FIG. 4b

FIG. 40

HIGH TOUGHNESS IRON BALLS AND PROCESS OF MAKING THE SAME DETAILED DESCRIPTION OF THE INVENTION The present invention relates to high toughness iron balls having excellent ductility, wear resistance, and heat resistance suitable for grinding use in ball mills, etc. The invention relates further to a process of making such excellent high toughness iron balls as mentioned above.

-Iron balls and cylpebs conventionally used for grinding cements, ores, coals, etc., suffer generally from quick wearing and damage by the influences of impact and heat and thus there has been an urgent need for improved grinding balls having excellent ductility and wear resistance capable of being used endurably even under severe grinding conditions.

As iron balls or cylpebs used for the above purposes, there have hitherto been employed cast iron series balls mainly represented by chilled balls made by casting in a sand mold or a metallic mold and'steel series balls mainly represented by steel balls made by die-forging or roll-forging. However, although the cast iron series balls are excellent in wear resistance, they are inferior in toughness and thus are apt to be readily damaged during use and further apt to be cracked by thermal influences. On the other hand, the steel series balls are excellent in toughness but they are inferior in wear resistanceand thus cannot endurably used for a long period of time.

An object of this invention is, therefore, to provide high toughness iron balls which are superior in ductility, wear resistance, and heat resistance and can be quite effectively and undurably used as iron balls for grinding in ball mills, etc.

Other object of this invention is to provide a novel and simple method of making the aforesaid high toughness iron balls.

Those and still other objects ofthis invention will become apparent from the following descriptions of this invention.

That is. according to the present invention, there are provided high toughness iron balls for grinding comprising white cast iron fine eutectic cementite structure formed by destroying the network structure of the eutectic structure in as-cast condition, said eutectic cementite having been dispersed and arranged concentric-spherically and uniformly in the ball.

The present invention provides also a method of making the high toughness iron balls as mentioned above which comprises casting the melt of iron having a chemical composition belonging to that of hypoeutectic pig iron into a white cast iron bar, subjecting, after heating, the white cast iron bar to hot plastic deformation working by forging at a temperature of 900C. to l,l2SC. to form white cast iron balls having such a structure that the finely destroyed eutectic cementite has been concentric-spherically and uniformly dispersed and arranged in the pearlitic matrix of the ball, rapid-cooling the balls, and then tempering the balls at a temperature range below the A, point of the cast iron or annealing the balls at a temperature range of 150C. to 250C.

White cast iron is ordinary brittle in the as-cast state which is apt to suffer segregations and internal defects. Therefore, for giving high toughness to such white cast iron, there have usually been employed a method in which the white cast iron is heat-treated for a long period of time to change the cast structure of it and a method in which magnesium, calcium, or an alloying element having the similar effect is added to the white cast iron to precipitate graphite therein and further the structure of the graphite is converted into a spheroidal or spindle-shaped graphite structure.

On the other hand, in the present invention the high toughness iron balls provided with excellent ductility as well as excellent wear resistance and heat resistance are prepared by subjecting white cast iron having a chemical composition range of hypo-eutectic pig iron to hot plastic deformation working to form white cast iron balls and then subjecting the cast iron balls to the heat treatment as mentioned above. Such excellent and epochal characters of the high toughness iron balls of this invention as iron balls for grinding have never been obtained by conventional methods of improving the characters of cast iron and further have never been seen in conventional chilled balls and steel balls.

The conditions for the hot deformation working and the heat treatment are important for producing the high toughness iron balls of this invention.

That is, in the first step or the hot plastic deformation working of producing the high toughness iron balls of this invention, the melt having a chemical composition belonging to that of hypo-eutectic pig iron is cast into a white cast iron bar having a simple sectional shape such as a round-type bar by means of a sand mold, a metallic mold, or a continuous casting machine and after heating, the white cast iron bar is subjected to a hot plastic deformation working by means of a hammer, a free forging machine, a die-forging machine, or a roll-forging machine at a temperature of 900C. to l,l20C. to form the balls of the white cast iron.

A cast block has ordinary a binary structure composed of austenite and eutectic cementite or a ternary structure composed of austenite, eutectic cementite, and a very small amount of ultra fined graphite. By the application of the aforesaid hot plastic deformation working, the hard and brittle cementite of the cast iron having a network structure in the as-cast condition is destroyed, cut, and fined to form a deformed eutectic cementite uniformly and concentric-spherically dispersed and arranged in the pearlitic matrix of the cast iron bail. Also, by the hot plastic deformation working, the segregations and other defects in the cast iron formed in the as-cast condition can be improved to such a state as giving no harmful effect or influence on the use of the iron balls of this invention,

Then, the cast iron balls are heat-treated under the following conditions for providing the high toughness iron balls of this invention. That is, the cast iron balls prepared by the aforesaid hot plastic deformation working are cooled rapidly by means of water cooling, oil cooling, spray cooling, air blast cooling, or air cooling and thereafter are tempered at a temperature range below the A point of the cast iron in accordance with the desired hardness for the iron balls or are annealed at a temperature range of C. to 250C. By the heattreatment, the mechanical and physical properties of the iron balls can be greatly improved to provide the high toughness iron balls of this invention.

In order to give wear resistance to ordinary steel balls, such an attempt has generally been employed that the steel balls are, after forging or roll-forging, subjected to a heat-treatment such as tempering to increase the surface hardness of the steel balls but by such a known technique the depth of the hardened layer is thin and thus in case of treating large steel balls, it is difficult to increase the hardness of the balls up to the deep inside portion. On the other hand, the surface hardness of chilled balls has generally improved, without applying a heat-treatment such as quenching and tempering, by precipitating a large amount of cementite having high hardness in the pearlitic matrix of the ball. The hardness drop in cross section of the chilled balls is less than the case of the steel balls but the chilled balls are brittle owing to the network structure of the eutectic cementite or the pro-eutectic cementite precipitated.

The high toughness iron balls of this invention have, however, such a specific structure that the eutectic cementite of network structure in the as-cast condition has been concentric-spherically, uniformly, and finely dispersed and arranged throughout the ball by the applications of the abovementioned hot plastic deformation working and heat-treatment.

The specific structure of the high toughness iron balls of this invention will further be explained by referring to the accompanying drawings, in which FIG. 1 is a graph showing the sectional hardness distributions in a conventional chilled ball (A) and a conventional steel ball (B) together with that in the high toughness iron ball (C) of this invention,

FIG. 2(a) is a sketch according to a micrograph showing the micro-structure of a conventional chilled ball and (b) is a sketch according to a photograph showing the macro-structure of the cross section of the chilled ball taken through the central portion thereof,

FIG. 3(a) is a sketch according to a micrograph showing the micro-structure of a conventional steel ball and (b) is a sketch according to a photograph showing the macro-structure of the cross section of the steel ball taken through the central portion thereof, and

FIG. 4(a) is a sketch according to a micrograph showing the micro-structure of the high toughness iron ball of this invention and (b) is a sketch according to a photograph showing the macro-structure of the cross section taken through the central portion thereof.

In the above figures (a), the magnification of the sketches of the micrograph is 50 and that of the sketches of the photographs is 1.

Now, as shown in FIG. I, it will be understood from the sectional hardness distribution of the high toughness iron ball of this invention that the hardness drop of the high toughness iron ball (C) of this invention from the surface to the central portion thereof is less than that of the conventional chilled ball (A), particularly that of the conventional steel ball (B).

The differences between the high toughness iron balls of this invention prepared by the process of this invention as mentioned above and the conventional chilled balls or steel balls will further explained in detail by referring to the results of observing the internal structures thereof.

As shown in FIG. 2(a) and FIG. 3(a), the microscopic structure of the chilled ball is different from the microscopic structure of the steel ball, that is, the wear resistance of the chilled ball is higher than that of the steel ball owing to the eutectic cementite (white part) present in the structure of the chilled ball. However, as

clear from FIG. 2(a), the state of the eutectic cementite precipitated in the structure of the chilled ball is the network state peculiar to the chilled ball and thus the chilled ball is essentially brittle as well as is low in resistance to the development of thermal cracking owing to the network structure of the eutectic cementite. Consequently, fine cracks are apt to form on the surface of the chilled ball, which results in causing readily undesirable fatigue fractures. Since the chilled balls are brittle as mentioned above, it is required for giving high toughness to the chilled balls to heat-treat the chilled balls for a long period of time to granulate the pearlite (black parts) and at the same time to spherodize the network structure composed of the eutectic cementite and pro-eutectic cementite groups (white parts) by cutting the network structure. However, even by applying such a heat-treatment, it is quite difficult to change fundamentally the structure of the eutectic cementite in the'as-cast state and thus the chilled balls are inevitably brittle as compared with the steel balls.

Also, as clear from the sketches of the macrophotographs of the cross sections of the balls through the central portions shown in FIG. 2(b) and FIG. 3(b), the surface hardened layer (white layer) of the steel ball formed by quenching is thin and accordingly although the steel balls may show high wear resistance at the beginning of grinding when the balls are used, the wear resistance thereof will be greatly reduced with the passage of time during use. On the other hand, in the chilled balls, the direction of solidifying structure is clearly observed in the direction of dead head as shown in FIG. 2(b). That is, the upward direction of the sketch shown in FIG. 2(b) is the direction of dead head and since the dendrite structure is developed converging to the dead head, the material quality of each portion in the inside of the chilled ball becomes inevitably ununiform.

FIGS. 4(a) and (b) show respectively the sketch of the microscopic structure of the high toughness iron ball of this invention and the sketch of the macroscopic structure of the cross section of the same ball through the central portion and the chemical composition of the iron ball is as shown in Example 1 of Table l, in which the crushing strength, the surface hardness, and the wear resistance of four examples of the high toughness iron balls of this invention are shown together with the chemical compositions of the balls and these of a conventional chilled ball and steel ball are also illustrated for the sake of comparison. As shown in FIGS. 4(a) and (b), the high toughness iron ball of this invention has such a specific or unique structure that the fine deformed eutectic cementite (white spots) formed by the aforesaid hot working have been uniformly dispersed and arranged in the tough pearlitic matrix (black parts) formed by the aforesaid heat-treatment. The white spots in FIG. 4 (b) show the deformed eutectic cementite.

The results of crushing strength test and wear resistance test of the high toughness iron balls of this invention and the conventional chilled ball and steel ball are shown in Table l as indicated above. The crushing strength test was conducted according to the crushing test of I18 3-1501, 9, 3. Also, the wear resistance test was conducted by charging the test balls having a diameter of 45 mm. in a ball mill containing kg. of silicon carbide particles of 60 mesh, rotating the mill at a Wear ratio to steel mean abrasion loss.

26218.; 50618 12011. SFrhe Eriihfi'strngth tests and the wear resistance tests shown in Table 1, it

will be understood that the high toughness iron balls of this invention are excellent in ductility by 3.6-4.0 times that of the chilled ball and l.4-l.6 times that of the steel ball and also in wear resistance by 1.2-2.7 times that of the chilled ball and 2.1-3.9 times that of the steel ball.

In addition, the high toughness iron balls of this invention used in the above tests and shown in the above table as Examples 1 to 4 were all prepared by the following manner. That is, the melt having the chemical composition shown in the table was melted in a 5-tons low-frequency induction furnace and cast by means of a horizontal-type continuous casting machine having a l-ton electric heating holding furnace into a round bar Others having a diameter of 32 mm. The cast iron bar thus prepared was introduced in a walking beam-type continuousreheating furnace at l,080C. and then cast iron balls having a diameter of 45 mm. were formed from the round bar of 32 mm. in diameter by means of a die- 2 forging machine at l,050C. The cast iron balls were immediately subjected to oil-cooling and then annealed for about 3 hours at 250C. in an annealing furnace to provide the high toughness iron balls of this invention.

Table l What is claimed is:

which comprises casting hypo-eutectic pig iron into a white cast iron bar, subjecting, after heating, the white 35 cast iron bar to hot plastic deformation working at a Chemical compos ition 0.007 0.06 0.012 0.005 essentially temperature of 900C. to l,l25C. to form white cast iron balls having a structure in which theeutectic cementite network is'converted to fine eutectic cementite uniformly and concentric-spherically dispersed 4 throughout the pearlitic matrix of the ball, rapid cooling the balls, and then tempering the balls at a temperature below the A point of the cast iron or annealing the balls at a temperature of 150C. to 250C.

2T A tough iron ball for grinding consisting of white cast iron having a fine, non-network eutectic cementite structure uniformly and concentric-spherically dispersed throughout said ball wherein the non-network eutectic cementite structure is formed by hot plastically deforming the network structure of the eutectic m 2 5.10 0.60 0.59 1.15 0.98 0.006 0.04 0.010 0.006 E1: 5 5.08 0.64 0.88 1.65 0.51 0.004 2.25 0.011 0.008 Ex 4 2.74 0.58 0.59 1.07 0.52 0.005 1.66 0.045 0.050

cementite in the as-cast condition of the white cast iron.

rate of 28 r. p. m. for 55 hours, and then measuring the 

1. A PROCESS OF MAKING HIGH TOUGHNESS IRON BALLS WHICH COMPRISES CASTING HYPO-EUTECTIC PIG IRON INTO A WHITE CAST IRON BAR, SUBJECTING, AFTER HEATING, THE WHITE CAST IRON BAR TO HOT PLACTIC DEFORMATION WORKING AT A TEMPERATURE OF 900*C. TO 1,125*C. TO FORM WHITE CAST IRON BALLS HAVING A STRUCTURE IN WHICH THE EUTECTIC CEMENTITE NETWORK IS CONVERTED TO FINE EUTECTIC CEMENTITE UNIFORMLY AND CONCENTRIC-SPHERICALLY DISPERSED THROUGHOUT THE PEARLITIC MATRIX OF THE BALL, RAPID COOLING THE BALLS, AND THEN TEMPERING THE BALLS AT A TEMPERATURE BELOW THE ACL POINT OF THE CAST IRON OR ANNEALING THE BALLS AT A TEMPERATURE OF 150*C. TO 250*C.
 2. A tough iron ball for grinding consisting of white cast iron having a fine, non-network eutectic cementite structure uniformly and concentric-spherically dispersed throughout said ball wherein the non-network eutectic cementite structure is formed by hot plastically deforming the network structure of the eutectic cementite in the as-cast condition of the white cast iron. 